An antenna or an antenna array (hereinafter collectively referenced as an “antenna”) shapes (by-design) radiated electromagnetic energy in specific patterns called beams. As such, the location of the center of the beam of an antenna also defines its pointing direction. For navigation satellites, it is useful to be able to point the beam of the navigation payload antenna to a specific location with increased precision as a jamming countermeasure, while for communications satellites, knowledge of the beam center permits communications to be conducted more efficiently. For example, more accurate pointing of the beam center of an antenna can facilitate communications at a lower power level or with improved throughput in comparison to communications conducted with an antenna having a beam center that is pointed with less precision.
The beam center of an antenna may be controlled by mechanically moving the antenna to point in a desired direction. Alternatively, in antenna arrays, the beam center may be controlled by appropriately commanding attenuators and phase shifters to alter the amplitude and phase, respectively, of the signals emitted by the different array elements. Although the beam center of an antenna can be altered, the ability with which the beam center of an antenna can be pointed in a desired direction depends upon the accuracy with which the current beam center may be determined. Additionally, the beam center of an antenna will generally vary with temperature due to, for example, thermal deformations of reflectors or feeds and/or the temperature stability of phase shifters, attenuators or the like. As such, even if the beam center of an antenna is appropriate at one point, the beam center of the antenna may change as the temperature changes such that by monitoring the current beam center of the antenna, the antenna can be appropriately repositioned to maintain its operational efficiency.
In order to determine the beam center of an antenna, the footprint of the antenna may be measured and the beam center may be determined based upon the antenna footprint and antenna location. Based upon the beam center that is determined by this process in comparison to the desired beam center, new settings may be transmitted to the antenna to drive the mechanical positioning system and/or the phase shifters, attenuators or the like so as to adjust the actual beam center to coincide with the desired beam center. For an antenna onboard a satellite, the antenna footprint is measured on the ground following transmission of the radio frequency (RF) signals through the earth's atmosphere. Since the earth's atmosphere is a thermally unstable dispersive media, the beam emitted by the antenna and, in turn, the antenna footprint on the ground may be altered by the atmosphere, thereby resulting in inaccuracies in the determination of the beam center. As such, the commands provided to the antenna in order to alter its pointing direction may be adversely affected with the beam center of the antenna still being offset from its desired position.
In order to provide improved performance for at least some antenna-based systems, such as improved location determination for navigation systems and efficient signaling for communication systems, it may be desirable to provide an improved technique for determining the beam center of an antenna. By more precisely determining the beam center of an antenna, the antenna could be repositioned in order to more accurately align the actual beam center of the antenna to the desired location of the beam center, thereby permitting signals to rise above jamming noise in navigation systems and signals to be communicated more efficiently in communication systems.